1. Field of the Invention
This invention relates in general to circuit arrangements for pulse Doppler radar receivers and in particular to a novel circuit. The invention relates to a circuit arrangement for a receiver of a pulse Doppler radar wherein a measuring signal for determining the mean frequency of disturbing low frequency received clutter signals are obtained from which a control signal is formed for a circuit which precedes the clutter filter and which serves to displace the clutter spectrum into the blocking ranges of the fixed clutter filter particularly for radar devices having a non-constantly scanning antenna and wherein a circuit is provided for determining the mean change in the clutter phase of consecutive radar periods from which the control signal for the displacement of the clutter spectrum is formed.
2. Description of the Prior Art
Prior art devices provide time placement delays for the clutter spectra, for example, due to a slow movement of the vehicle which carries the radar device or due to differing conditions in the relevant directions of an antenna during scanning. If counter measures are not taken, such displacements in the clutter spectrum give rise to an increase in disturbing received signals in the pass range of the moving target clutter filter. For this reason, processes have been developed which allow an adaptive displacement of the time-variant clutter spectrum to be effective relative to the blocking range of a clutter filter. Examples are disclosed in the article by Voles in Proceedings of the IEE, Vol. 122, No. 7 of July, 1975 at pages 689-692 and in the IEEE Transactions AES, of November, 1973 at pages 950-953.
An adaptive displacement of these types of the time-variant clutter spectrum, has the advantage that no mean error occurs in the determination of the phase difference required for the displacement between two received clutter echoes and there is no dependence of the variance of the error upon the mean clutter frequency.
The disadvantages of these prior art known regulating processes is that "build-up processes" occur which are likely when, for example, a scanning process commences or when a non-constant scanning of the area by the radar antenna takes place in the so-called step-scan operation. Non-constant space scanning of this type occurs in particular when phase control radar antenna are used. The first pulses cannot be used for analysis and are lost.
The adaptive regulation process described in the publication IEEE Eascon Conference Record, Washington, D.C. September, 1973 at pages 170-176 describes operation with a constant all-around antenna which has this disadvantage of the build-up process, in other words, the time until full sensitivity is reached can comprise up to ten radar periods and the associated pulse echo signals are lost in this analysis.
The article IEEE Transactions on Aerospace and Electronic Systems AES-9, Vol. 2, March, 1973 discloses adaptive filters for a clutter suppression. The parameters in this article which modify the filter characteristics are varied depending upon the clutter spectrum in a manner such that these undesired signals are suppressed. As this necessitates a squaring of the clutter signals and occupies a specific length of time, delay devices which compensate for this time delay are provided in the signal transmission path. The use of adaptive filters is only possible with a viable outlay if these filters are of simple construction as only then will a few parameters need to be modified. A displace of the clutter spectrum is advantageous in comparison to an adaptive filter because no change is necessary in filter parameters and, therefore, it is possible to use high grade moving target filters of a correspondingly complicated construction.